1. Field of the Invention
The invention relates to the field of methods and apparatus for the delivery of compressed gas for, inter alia, industrial or pipeline uses.
2. Description of the Prior Art
Nitrogen, air or other gas is used to service pipelines, tanks, or other items for the purpose of performing maintenance or some other operations. Typically dry air, nitrogen, or other gas is used, for example, to inert or dry pipelines 16 (FIG. 1), or to propel pigs for cleaning or inspecting pipelines 16. This dry air or nitrogen usually comes from either large motor or engine driven compressors 1 (FIG. 1), from nitrogen gas generators 1, which have a limited flow rate capability, or from liquid nitrogen which must be delivered to the site in sufficient quantities for the operation and then be pumped and vaporized with motor or engine driven pumps and heating equipment at the rate required by the application at hand. Nitrogen is the preferred gas to use for these applications due to its inherent nature of being relatively inert and very dry.
Time and cost are two major factors in determining what process to use in performing any of the operations. The fastest way to perform any of the operations is to obtain the highest flow rate of gas possible. In the current state of the art, this usually means using liquid nitrogen, which is the most expensive, equipment intensive and logistically difficult means.
The use of liquid nitrogen has been the preferred method of delivering gas for these applications because very high flow rates can be achieved at pressure required. When this method is used, the amount of gas must be estimated so that sufficient liquid nitrogen is available to complete the job. The disadvantages of this approach are the expense and logistics required for the supply and transportation of the liquid nitrogen, and the special equipment required to pump and vaporize the cryogenic medium.
The use of electric motor or engine driven compressors 1 to deliver the required gas (plus a nitrogen generator if necessary), although capable of delivering unlimited amounts of gas at a relatively low cost, is limited as to the rate at which the gas can be delivered and therefore increases the time required to perform the job.
Many of the operations that require a gas supply take place in hazardous locations (NEC Class 1 Division 2, or Zone 1 or 2) where the use of engines or electric motors 1 is discouraged, restricted, or require extensive safety provisions. In these cases both the use of the liquid nitrogen equipment and the compressors 1 present a potential problem that can usually only be remedied with expensive modifications to the equipment, or by using special equipment.